Acrylamide is in widespread use, for example, as a starting material for the production of various polymers for use as flocculants, stock additives, polymers of petroleum recovery, and so forth.
As a process for the production of such acrylamide, a method has heretofore been known which comprises reacting acrylonitrile with water by the use of a catalyst comprising copper in a reduced state. This method, however, suffers from various problems, for example, with respect to complexity in the preparation of the catalyst, difficulty in regeneration of the used catalyst, and complexity in the separation and purification of acrylamide formed. Furthermore, it is desirable to produce acrylamide under moderate reaction conditions, because compounds containing double bonds in the molecule, such as acryalmide, are readily polymerizable.
It has therefore been considered very desirable to establish a process for producing acrylamide by hydrolyzing acrylonitrile with microorganisms under moderate conditions.
It has long been known that microorganisms having nitrilasic activity are effective in hydrolyzing acrylonitrile to yield acrylamide. As such microorganisms, those belonging to the genera Bacillus, Bacteridium in the sense of Prevot, Micrococcus and Brevibacterium in the sense of Bergey, etc., are known (e.g., see, for example, U.S. Pat. No. 4,001,081). It has also been found that microorganisms belonging to the genera Corynebacterium and Nocardia are useful for the hydrolysis of acrylonitrile (e.g., see U.S. Pat. No. 4,248,968).
In producing acrylamide from acrylonitrile by the use of such microorganisms, acrylonitrile is brought into contact with the microorganisms or immobilized cells thereof prepared by immobilizing the cells with polymer gels, in an aqueous medium such as water, a physiological saline solution, and a phosphate buffer solution. Recently, a batchwise or continuous column method using granulated immobilized cells has been in widespread use, for the purposes of the prevention of elution of impurities from the cells, for improving separation of the cells from a reaction solution, for repeated utilization of the cells, and for increasing the stability of enzymes. Such methods using granulated immobilized cells are advantageous from an economic viewpoint. Therefore, a process has been proposed for the production of acrylamide by a continuous column reaction using immobilized microorganism cells which are prepared by entrapping the cells with a gel of polyacrylamide or the like (e.g., see U.S. Pat. No. 4,248,968).
In the processes as proposed above, however, the use of a physiological saline solution, phosphate buffer solution, or the like as an aqueous medium results in the introduction of large amounts of sodium chloride, phosphates and the like into the aqueous acrylamide solution formed, which is not desirable from the viewpoint of the quality of the desired product. In particular, in the production of acrylamide-based polymers having a high molecular weight, the presence of phosphates in acrylamide is liable to cause water-insolubilization of the polymers formed. For the removal of such salts, therefore, it is essential to apply post-treatments such as an ion exchange treatment. This leads to the loss of the advantage that a high quality acrylamide aqueous solution can be prepared without any special purification step, which is a feature of the method of producing acrylamide by an immobilized cell method. Thus, the advantage of the immobilized cell method as an inexpensive method for the production of acrylamide is lost.
On the other hand, if the physiological saline solution, phosphate buffer solution, or the like is not used as the aqueous medium, the immobilized cells swell in the course of the hydration reaction, and the enzymatic activity of the cells is rapidly lost. Furthermore, in the case of the column reaction, when an aqueous solution of acrylonitrile is passed through a column which is packed with cells conventionally immobilized with polyacrylamide, the immobilized cells in the column swell in a short time after the start of the hydration reaction, as a result of which efficient operation of the method becomes impossible.
Although the reason why the immobilized cells swell during the hydration reaction is not completely clear, it is believed to be due to the repulsion force generated among negatively charged cells in passing a substrate solution thereover, and the difference in osmotic pressure between the outside and inside of the immobilized cells, resulting from the difference in concentrations of acrylonitrile and acrylamide between the outside and inside of the immobilized cells, which occurs when acrylonitrile enters into the immobilized cells and is converted (hydrated) into acrylamide, and the thus-formed acrylamide migrates out of the immobilized cells. Furthermore, it is also believed that the deterioration of the enzymatic activity by the swelling phenomenon is due to the facts that the enzyme is liable to leak out of the immobilized cells due to swelling, and that the stable conformation in normal cells in which the enzyme is not swollen cannot be maintained. Therefore, it is believed that when the reaction is carried out in an isotonic medium such as a physiological saline solution, a phosphate buffer solution, etc., no great difference in osmotic pressure between the outside and inside of the immobilized cells is created, and therefore the swelling of the immobilized cells can be prevented while at the same time the enzyme can be maintained in a stable condition.